Visible light responsive photocatalysts are expected to be applied, as photocatalysts which can utilize visible light which is contained in large amount in sunlight, to photodecomposition of organic substances and production of hydrogen by photolysis of water. Especially, photocatalysts for splitting water to produce hydrogen have drawn attention as photocatalysts used for a generation method for hydrogen by utilizing a renewable energy. Consequently, the demand for photocatalysts for splitting water having a high activity has increased year after year.
A photocatalyst consisting of a metal nitride containing a metal and nitrogen in the crystal lattice thereof, or a photocatalyst consisting of a metal oxynitride containing a metal, oxygen, and nitrogen in the crystal lattice thereof is known as a visible light responsive photocatalyst having high activity for splitting water (see, for example, Patent Literature 1 and Non Patent Literature 1).